1. Field of the Invention
The present invention relates to subterranean cementing operations, and more particularly, to cement compositions comprising microspheres having improved mechanical properties, processes of preparing such compositions, and methods of using such compositions in subterranean cementing operations.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in subterranean operations, particularly subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby pipe strings such as casings and liners are cemented in well bores. In performing primary cementing, hydraulic cement compositions are pumped into the annular space between the walls of a well bore and the exterior surface of the pipe string disposed therein. The cement composition is permitted to set in the annular space, thereby forming an annular sheath of hardened substantially impermeable cement therein that substantially supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions also are used in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks in holes in pipe strings, and the like.
Subterranean formations transversed by well bores are often weak and extensively fractured. Accordingly, the loss of circulation of cement during placement is of great concern, and often calls for reducing the density of the cement as much as possible to accordingly reduce the hydrostatic pressure. A traditional means of reducing the density of the cement has been to increase the cement's water content, because, generally speaking, the higher the water content, the lighter the cement. However, this method may be problematic because the resultant cement often lacks the desired strength, especially in light of the stresses to which the cement will be subjected.
Cement compositions containing lightweight spherical or substantially spherical compounds have been developed as an alternative means of reducing the density of a given cement composition. Some compositions comprise microspheres, which typically comprise cenospheres, glass hollow microspheres, or ceramic hollow microspheres. Cenospheres are hollow spheres primarily comprising silica (SiO2) and alumina (Al2O3), and are filled with gas. These are a naturally occurring by-product of the burning process of a coal-fired power plant. The addition of conventional glass hollow microspheres and ceramic hollow microspheres reduces the density of the cement composition and produces a lightweight cement composition, e.g., a cement composition having a density less than about 13 pound per gal.
Cement failures can be particularly problematic in lightweight cement compositions containing a high proportion of microspheres, as these compositions have demonstrably reduced strength. This problematic reduction in strength is believed to be attributable to the combination of reduced content of cementitious materials and increased void space, both of which are caused by the addition of the hollow microspheres to the cement composition. Failure of cement within the well bore can result in radial or circumferential cracking of the cement as well as a breakdown of the bonds between the cement and the pipe or between the cement sheath and the surrounding subterranean formations. Such failures can result in at least lost production, environmental pollution, hazardous rig operations, and/or hazardous production operations. Another undesirable result is the presence of pressure at the well head in the form of trapped gas between casing strings.
To successfully meet the subterranean challenges to which a cement composition may be exposed, a low-density cement composition must develop high bond strength after setting and also have sufficient elasticity and ductility to resist loss of pipe or formation bonding, cracking and/or shattering as a result of all of the stressful conditions that may plague the well, including impacts and/or shocks generated by drilling and other well operations.